Conversion of a boroxazolidone, a 5-membered cyclic compound derived from an &agr -amino acid, into an &agr -amino acid by an entirely new ring-opening reaction (Umezawa reaction). (I) → (II) (In the formulae, R and R' each means optionally substituted alkyl or aryl.) The reaction is a novel and effective ring opening/protective-group elimination reaction by which a boroxazolidone can be easily converted to a corresponding compound (&agr -amino acid) under milder conditions at a high conversion.